Carbon-based piezoresistive nanomaterials are widely used for the fabrication of flexible sensors. Although our previous work demonstrated that an electrical breakdown (EBD) process can endow a graphene/polyimide (G/PI) composite with piezoresistivity, the formation of EBD-induced electrical traces with high consistency in bulk nanocomposites remains a technical challenge. With the aim of developing highly sensitive flexible strain sensors using a batch fabrication process, we introduce herein a microscale EBD (μEBD) method to form localized piezoresistors with diverse shapes in a G/PI thin film. The results of scanning electron microscopy, Raman spectroscopy, and electromechanical tests indicate that high piezoresistivity is derived from the high porosity of the carbonized conductive traces generated by the μEBD process. The gauge factor of the μEBD-treated G/PI strain sensor is over 20 times greater than that of the as-prepared G/PI film, and the sensitivities of the strain sensors can be tuned by varying the applied current in the μEBD process. We also demonstrate the potential applications of μEBD-treated G/PI strain sensors in the fields of finger gesture detection, sound pressure measurement, and airflow sensing.